In many cases, an underpants-type disposable diaper is formed of a liquid-permeable surface sheet, a back side sheet, and an absorber interposed between them, including gathers for areas around legs at both sides thereof, an inner body formed so as to cover from a back side through a crotch portion to a ventral side, and an outer sheet bonded to an outer surface of the inner body and formed so as to cover the abdomen portion and the crotch portion, with an underpants-type structure in which a back body part and a front body part of the outer sheet are each joined together on both sides by heat sealing or ultrasonic sealing to form side seal sections, whereby a waist opening and leg openings are previously formed.
Such an underpants-type disposable diaper is unclothed after excretion or the like by tearing off the back body part and the front body part from each other at the side seal sections thereof so as to be removed from the body. As a result, the side seal section is required to provide not only a seal strength enough not to be broken during the use of the diaper but also ease of tearing off after the use of the diaper. Various techniques, accordingly, have been conventionally suggested (see, for example, Patent Documents 1 to 10). For example, techniques described in Patent Documents 1 and 2 suggest seal patterns designed in view of the number of material sheets in the side seal sections.
Especially, a dot pattern in which dot-shaped welded parts are alternately arranged in a serpentine pattern, as shown in FIG. 6 of Patent Document 1 or 2 or Patent Document 5, has an advantage, i.e., the tearing is more easily performed in the dot pattern than a horizontal stripe pattern in which welded straight lines each disposed along in a transverse direction (in the form of a rectangle whose long sides are disposed in a transverse direction, and the like) are arranged in a vertical direction, as shown in Patent Document 4.